The present invention generally relates to non-destructive testing of materials and, more particularly, to the generation of reference standards for acoustic non-destructive testing of porous materials.
Composite laminate structures have been finding increasing application in commercial passenger aircraft, such as the Boeing 787 passenger jet in which large areas of the wing, empennage, and fuselage are constructed of carbon laminates. Portions of an aircraft that are considered to be a part of the primary structure have stringent inspection requirements associated with them. Therefore, those portions of the primary structure constructed of porous materials such as carbon laminate must pass rigorous testing. As a result, methods for inspecting these porous materials must reliably detect and quantify delaminations, foreign material inclusions, and porosity in the materials.
In the case of porosity, quantitative evaluation is a complex problem. Porosity is an allowable condition up to a certain extent. Measuring porosity nondestructively requires the use of ultrasonics, wherein the acoustic response of the part being tested is compared to porosity calibration standards (i.e., reference standards) of similar thickness containing known levels of porosity. FIG. 1 is an image 100 of an acoustic impression of a test material, such as a composite material. FIG. 1 illustrates ultrasonic data from a composite laminate with porosity. The image 100 is garnered using a non-destructive ultrasonic apparatus, such as a transducer, a through transmission transducer, a pulse echo transducer or an eddy current transducer. FIG. 1 shows two areas of the laminate having differing porosity readings. The first area 102 shows an area of high porosity. The second area 104 shows an area of low porosity.
Currently, porosity calibration standards are produced using an “exact analog” method wherein actual composite parts containing varying levels of porosity are produced. The ultrasonic attenuation of these porosity calibration standards is used to produce curves of attenuation versus porosity level. These attenuation curves are then compared to readings taken from the parts being tested. This approach, however, does not come without its problems.
One problem with this approach is that the attenuation curve represented by the porosity calibration standards can only be used with a particular inspection system because different instruments have different pulser and receiver characteristics, and transducers have different frequency bandwidths. In practice, this means that two ultrasonic testing systems using the same porosity calibration standards may produce different results. For example, a material having two percent porosity may register as 12 dB on a first ultrasonic testing system, but only 8 dB with a second ultrasonic testing system. Thus, if the porosity calibration curve for the first ultrasonic testing system is used by the operator of the second ultrasonic testing system, that operator would under-call the true porosity level in the part being tested. In order to ensure that different systems achieve the same results, the “exact analog” porosity calibration standards are produced in multiple sets—one set for each ultrasonic testing system—and shipped to each tester. This approach, therefore, requires the fabrication of multiple porosity calibration standards.
FIG. 2 is a chart 200 of an x-y plot showing a set of curves 212-217 indicating attenuation vs. thickness for a through-transmission technique using a conventional ultrasonic inspection reference standard. Attenuation is shown along the y-axis 202 measured in decibel (dB), while composite material thickness is shown on the x-axis 204 measured in inches. The attenuation is a decrease in intensity of a sound wave as a result of absorption of energy and of scattering out of the path of a receiving transducer.
Each of the six curves shown in the chart 200 represents the attenuation readings from a separate ultrasonic testing system, wherein all six ultrasonic testing systems used the same target composite part. Curve 212, for example, shows six separate data points representing readings garnered from an ultrasonic testing system. The curve 212 is extrapolated using the six readings. Each of the curves 213, 214, 215 and 216 also represent curves that were extrapolated using six separate data points representing readings garnered from a separate ultrasonic testing system. Curve 217 shows three separate data points representing readings garnered from another ultrasonic testing system. The curve 217 is extrapolated using the three readings.
As shown in the chart, each curve 212-217 represents the readings garnered from a different ultrasonic inspection system or inspection method but using the same test composite part. The reason for the difference in allowable attenuation is due to equipment variability from system to system or inspection method to inspection method. Examples of equipment variables that affect the attenuation curves include transducer type, transducer frequency, transducer diameter, transducer focal length, water column diameter in squirter systems, pulser type, system bandwidth, system dynamic range, and index dimension.
This approach requires the production of multiple porosity calibration standards. The standards are currently made by curing composite laminates with incremental cure variations intended to produce increased porosity. Once this is complete, the laminates are scanned with through-transmission ultrasonics. The process often produces laminates with spatially uneven levels of porosity, such that a relatively uniform area must be selected for independent verification of porosity level. The independent verification process involves comparison with master standards, if they exist, that have been inspected by the same system. If master standards do not exist, the part must be sectioned as close as possible to the area of interest and destructively examined for porosity level. The area of interest is then cut into coupons (or small rectangular patches) and engraved with the identified porosity level. This process must be repeated for each thickness that is necessary to produce data points in an attenuation curve, with at least eight thicknesses required, sometimes more.
While the technique described above has been used in production applications to make sets of porosity calibration standards, it is not considered satisfactory for the parallel problem of evaluating porosity in composite repairs when an aircraft is in service. Producing sets of porosity calibration standards for every airline and maintenance base is not practical due to time and money constraints. Faced with this problem, another acoustic substitute approach emerged. This method uses a phenolic step wedge as a porosity calibration standard that replicates the attenuation response from a threshold level (e.g., 4 percent) of porosity, which is the threshold rejectable level for porous materials. In the field, the inspector calibrates the ultrasonic testing system on the porosity calibration standard (i.e., the wedge) and then tests the part being repaired. If the reading from the part being repaired is lower than the reading from the standard, the part is rejected. Otherwise, the part is deemed acceptable for use.
The inspection using the approach above is a “go/no-go” decision. That is, the part being repaired is either acceptable (porosity below threshold level) or rejected (porosity above threshold level). In many cases the tester must know the actual porosity level of the part. With the go/no-go standard, the tester is not capable of providing this information.
Yet another acoustic substitute approach that emerged involved the use of circular channels embedded in blocks of composite materials. This method uses a composite block with the channels as a porosity calibration standard that attempted to replicate the attenuation response from an aircraft part with a given porosity value. This approach, however, was not successful, as the channels were not able to adequately replicate the attenuation response from an aircraft part with a given porosity value.
As can be seen, there is a need for an alternative approach to building porosity calibration standards for composite materials in order to reduce the cost of design, manufacturing, and qualification. Furthermore, there is a need for a method that enables the mass production of porosity calibration standards that can be used for ultrasonic inspection of composite materials due to the growing industrial application of composite materials and, therefore, an increased volume of composite parts to be inspected using ultrasonic inspection techniques.